The present invention relates to image sensors and, more particularly, to such image sensors having black reference pixels located in a non-intrusive and more efficient position.
Currently used imaging sensors have black reference pixels located in the area immediately surrounding the active pixels. As is well known in the art, such black reference pixels are used for calibration of the imaging processing chain. These black reference pixels may become contaminated by lateral diffusion in which red light generated electrons from the active pixel area contaminate the first few rows and columns of the black reference pixels. This results in inaccurate reproduction of the desired image.
Currently used CCD imaging sensors compensate for lateral diffusion by adding additional rows and columns of pixels in which these buffer pixel rows and columns absorb the lateral diffused energy. This solution requires the use of a secondary light shield that prevents illumination of the buffer pixel rows and columns. An alternate solution is to increase the number of black pixels beyond the amount normally needed. The extra black pixels are used to absorb the energy from lateral diffusion. In this case, a secondary light shield is not necessary.
In CMOS image sensors, several layers of an inter-leaved material (usually metal) are used to shield light from the black reference pixels. These inter-leaved layers, however, allow light to leak therethrough and illuminate the black pixels, resulting in black pixels that are light sensitive. Obviously, this is an undesirable phenomenon.
Although the currently known and utilized methods and apparatus for lateral diffusion compensation are satisfactory, they include drawbacks. Adding buffer pixel rows and columns uses significant areas of the silicon. Similarly, additional rows and columns of black reference pixels also uses significant areas of the silicon, which is not cost-efficient.
In CMOS imagers the inter-leaved layers of material, used for black shielding, are not totally opaque to light. The solution to this problem is to add a secondary light shield, which in turn increases the cost.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in an image sensor comprising: (a) an active pixel area for image capture; (b) one or more black pixel areas disposed in a pre-determined, significant spaced apart distance from the active pixel area; and (c) a light shield to prevent light from illuminating the black pixel areas.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
The present invention has the advantage of improved performance black reference pixels in electronic imagers. This is accomplished through the use of a coverglass secondary light shield and geographically moving the black pixels a significant distance from the imaging pixels. This invention can be applied to all solid state imagers.